1. Field of the Invention
This invention relates to lighting and display devices, and in particular, it relates to a light source device and projection system using the light source device.
2. Description of the Related Art
In a conventional projection system, three single-color images are projected onto a screen to form a single full-color image by integration effect of viewer's eye. Three separated modulators are often used to simultaneously modulate three primary-color images. For example, the light from a white light source is separated by a set of appropriate dichroic filters into three single-color lights, which are modulated respectively by three separated liquid crystal display (LCD) panels or three digital micromirror device (DMD) to generate three primary images. Alternatively, a single modulator with sequential color light could sequentially generate primary-color images. If the color light sequences through the three primary colors are fast enough, the viewer will not see the sequential color images but instead will see a full-color image. A sequential color light source is formed by combining a white light source and a filter means such as color filter.
A color wheel is typically a disk-shaped assembly of dichroic filters. The light comes from the white light is focused on the color wheel and filters, which are transmissive or reflective, to filter the input white light into single color lights. At least one color filter is included for each of primary color segment. FIG. 1 shows the transmission spectra of the three filters of a color wheel. The locations of the color wheel and the light source are relatively fixed so that the focus point of the white light on the color wheel is stationary. Spinning the color wheel so that each of the filters passes through the point at which the white light strikes the wheel generates a sequential primary-color light beam. The color wheel is typically spun fast enough to provide at least one primary color for each primary in each frame of a video image to avoid the detection of the sequential color nature on the screen by the viewer.
While a sequential color display system typically costs less than a simultaneous color system, images created by a sequential filtered color display system are not as bright as images created by simultaneous color display systems using the same light source. This is because at any given time only a portion the light generated by the light source is used to form the image. Additionally, when a sequential filter such as color wheel is used, the light during the filter transitions, typically called spoke light, will be a varying mixture of the two filters being changed in and out. This mixed-color light cannot be used by the same way of the pure color light since using the mixed color light would adversely affect the color purity of the created image.
A solution to the spoke light is to turn off the modulator during the spoke period, which refers to the time period of the filter transition. When the spoke period is turned off, light from the color wheel still passes to the light modulator, but just isn't used to contribute to the image brightness. Because the total spoke periods often take a proportion of 10%˜20% of each frame period, turning off the spoke period will result in a dramatic degradation of brightness.
Several methods and apparatuses are presented to solve this problem. Patent U.S. Pat. No. 6,324,006 disclosed a method to utilize the spoke period for each pixel by different spoke-bit to keep the image artifacts below a perceptible threshold. US 2005168454 disclosed another method in which some of the spoke period could be used to contribute to the image when the color wheel in this system includes at least one neutral density (ND) segment. US 2006192734 disclosed another method to turn on the spoke periods while at least one primary-color is controlled to be decreased to compensate the chromatic artifact duo to the spoke period light.
These prior art display systems and methods have been used to maximize the image brightness by spoke light recapture (SLR) mechanism. The white-light source they used, however, is usually a UHP lamp, which typically has a short lifetime of 3000 hours. Additionally, the UHP lamp contains mercury which is not environmental-friendly. Solid-state light sources are developed in recent years, which have a long lifetime of more than 50000 hours, and contain no harmful ingredients. Therefore, Solid-state light sources such as light emitting diode (LED) can potentially replace UHP lamps.
As mentioned above in a single-modulator system a sequential color light source is required to sequentially generate three primary-color images. Different from the architecture of combination of white-light source and color wheel, three single-color solid-state light sources, such as red, green and blue LED sources, are combined by a set of dichroic filters in wavelength domain to act as a full-band light source. FIG. 3 illustrates a schematic view of single-modulator projection display system which makes use of solid-state light sources as an illuminator.
In FIG. 3, reference symbols 501, 502 and 503 refer to red (R), green (G) and blue (B) LED sources respectively. Dichroic filter 504 is transmissive to green and red light and reflective to blue light, while another dichroic filter 505 is transmissive to green and blue light and reflective to red light. These two dichroic filters combine the three primary lights by transmitting and reflecting them to a same exit to generate a full-band light source, whose spectrum is shown in FIG. 7, in which reference symbols 1001, 1002 and 1003 refer to blue, green and red color components respectively.
In FIG. 3 the combined full-band light is collected and focused onto the modulator 506 by a set of lens to generate modulated light, which is projected by a projection lens 507 to a screen (not shown). To generate sequential color light, a controller 512 is also used to respectively control light sources 501˜503 to turn on and off in sequence, based on a sync signal from the modulator 506 through the connection 508.